1. Field of the Invention
This invention relates to devices and methods for handling printed circuit boards during manufacturing processes. More particularly, this invention relates to an apparatus and methods for leveling the upper surface of a printed circuit board on the tooling table of a pick-and-place machine.
2. Background Information
Pick-and-place machines are commonly used to mount components, such as integrated circuit chips, capacitors, and resistors, onto printed circuit boards (PCBs). The use of pick-and-place machines to increase the speed with which components are surface-mounted to PCBs is well known in the art. Pick-and-place machines remove components from a stored position or a feeder, align the components with the PCB, and place the components in the proper position on the PCB. Pick-and-place machines are used to populate both single-sided and double-sided PCBs.
The pick-and-place machine contains a device, such as a robotic arm, to move components from stored positions to the PCB surface. The robotic arm typically contains a gripping mechanism, such as a suction nozzle head, to grip components in the stored position and transport them to the PCB. Some pick-and-place machines have robotic arms that move in the X and Y-axes only, while other pick-and-place machines have robotic arms that move in the Z-axis or in a combination of all three axes.
To ensure the proper placement of components on the PCB, the alignment of the PCB with the pick-and-place machine, in particular the robotic arm, must be calibrated. The PCB is secured to the tooling table of the pick-and-place machine to ensure a fixed alignment of the PCB on the table. The tooling table may then be adjusted in the X, Y, and Z axes and examined by eye for proper positioning. Proper positioning of the PCB on the tooling table allows the robotic arm head to pick up a component from the stored position, scan the PCB, and then place the component in the proper position on the PCB.
The use of part decryption generators (PDGs) to populate PCBs with components is well-known in the art. A PDG is a program that instructs the pick-and-place machine where to place specific components on the PCB surface. The PDG also directs a camera or similar scanning device, which may be mounted on the robotic arm head, to scan the PCB surface. The PDG uses the results of this scan as a location guide for placement of components on the PCB surface.
When the PDG uses the robotic arm head to scan the PCB, it reads a Z-level where it contacts the PCB and assumes this level to be constant over the entire upper surface of the PCB. If the upper surface of the PCB is not at a fixed Z-level, i.e., is uneven due to warping, thickness variations or other abnormalities, the PDG will improperly place components on the PCB. If the PDG assumes a constant Z-level and some areas of the PCB are above or below that level, components in those areas will not be placed at the correct Z depth. With current pick-and-place equipment, an average of about ten percent of components are misplaced if the upper surface of the PCB is not level, and up to seventy-five percent of components may be misplaced in a severe case. The improper placement of components on PCBs not only causes problems with the success of PCB production, but the nozzle heads of the pick-and-place machine's robotic arm may be damaged by overdriving parts into PCBs.
The pick-and-place machine industry has provided mounting pins and clamps to hold PCBs to the tooling tables. The mounting pins are commonly solid pin dowels that thread into fixed locations in the tooling table. The height of the mounting pins may be adjusted by threading the pins into or out of the tooling table. The PCB may then be placed upon the mounting pins. Some pick-and-place machines provide clamps to pinch the PCB from the top above each of the mounting pins to support the PCB. Other pick-and-place machines come with a suction securing system. In these systems the mounting pins have holes drilled in them. The bottom side of each mounting pin is connected to a vacuum supply to create suction forces on the upper side of the mounting pin where it contacts the PCB. The suction forces hold the PCB down to the mounting pins, which in some cases may permit correction of warping.
There are a number of problems that result from securing PCBs to tooling tables with mounting pins. Double-sided PCBs contain components on the bottom side after population of one side with components. Because the mounting pins may only be placed at fixed positions in the tooling table of the pick-and-place machine, the mounting pins may contact the PCB where fragile components, such as capacitors, have been mounted. This contact may damage the components.
Another problem with the use of mounting pins is the difficulty in flattening out the upper surface of PCBs that are warped. Mounting pins secure the PCB by grasping the PCB from the top and from the bottom. The use of a plurality of mounting pins to support a PCB may not flatten out the PCB and remove the warping from its upper surface. Even a plurality of properly placed mounting pins may not be able to remove the warping from a large PCB because it is difficult to stretch or flatten out the PCB with mounting pins that provide only vertical support at limited points of contact. The use of mounting pins may cause some areas of the PCB to sag so that the upper surface of the PCB is not flat.
Yet another problem with the use of mounting pins to support PCBs upon tooling tables is the difficulty in achieving a repeatable flat surface for the PCB. If the pick-and-place machine is used to populate a different size or type of PCB, the location and height of the mounting pins will need to be modified to accommodate the new PCB. The mounting pins will have to be positioned and adjusted from scratch the next time the first line of PCBs is to be populated. A related problem results if one pick-and-place machine is set up for population of a given PCB line and it is desired to use a different pick-and-place machine for population of that PCB line. The mounting pin placement on the new pick-and-place machine will have to be adjusted from scratch to achieve the proper Z-level.
A need exists for a method and apparatus to provide a level upper surface for PCBs during population of components using pick-and-place machines that is repeatable, may be used for different pick-and-place machines with minimal effort, and that does not damage components on double-sided PCBs.